Behavioral economics and drug choice: effects of unit price on cocaine self-administration by monkeys

An automated group-housed oral fentanyl self-administration method in mice

RATIONALE AND OBJECTIVES

Social factors play a critical role in human drug addiction, and humans often consume drugs together with their peers. In contrast, in traditional animal models of addiction, rodents consume or self-administer the drug in their homecage or operant self-administration chambers while isolated from their peers. Here, we describe HOMECAGE ("Home-cage Observation and Measurement for Experimental Control and Analysis in a Group-housed Environment"), a translationally relevant method for studying oral opioid self-administration in mice. This setting reduces experimental confounds introduced by social isolation or interaction with the experimenter.

METHODS

We have developed HOMECAGE, a method in which mice are group-housed and individually monitored for their consumption of a drug vs. a reference liquid.

RESULTS

Mice in HOMECAGE preserve naturalistic aspects of behavior, including social interactions and circadian activity. The mice showed a preference for fentanyl and escalated their fentanyl intake over time. Mice preferred to consume fentanyl in bouts during the dark cycle. Mice entrained to the reinforcement schedule of the task, optimizing their pokes to obtain fentanyl rewards, and maintained responding for fentanyl under a progressive ratio schedule. HOMECAGE also enabled the detection of cage-specific and individual-specific behavior patterns and allowed the identification of differences in fentanyl consumption between co-housed control and experimental mice.

CONCLUSIONS

HOMECAGE serves as a valuable procedure for translationally relevant studies on oral opioid intake under conditions that more closely mimic the human condition. The method enables naturalistic investigation of factors contributing to opioid addiction-related behaviors and can be used to identify novel treatments.

Beyond simple tests of value: measuring addiction as a heterogeneous disease of computation-specific valuation processes

Addiction is considered to be a neurobiological disorder of learning and memory because addiction is capable of producing lasting changes in the brain. Recovering addicts chronically struggle with making poor decisions that ultimately lead to relapse, suggesting a view of addiction also as a neurobiological disorder of decision-making information processing. How the brain makes decisions depends on how decision-making processes access information stored as memories in the brain. Advancements in circuit-dissection tools and recent theories in neuroeconomics suggest that neurally dissociable valuation processes access distinct memories differently, and thus are uniquely susceptible as the brain changes during addiction. If addiction is to be considered a neurobiological disorder of memory, and thus decision-making, the heterogeneity with which information is both stored and processed must be taken into account in addiction studies. Addiction etiology can vary widely from person to person. We propose that addiction is not a single disease, nor simply a disorder of learning and memory, but rather a collection of symptoms of heterogeneous neurobiological diseases of distinct circuit-computation-specific decision-making processes.

Behavioral economic analysis of the effects of N-substituted benztropine analogs on cocaine self-administration in rats

RATIONALE AND OBJECTIVES

Benztropine (BZT) analogs and other atypical dopamine uptake inhibitors selectively decrease cocaine self-administration at doses that do not affect responding maintained by other reinforcers. Those effects were further characterized in the current study using a behavioral economic assessment of how response requirement (price) affects reinforcers obtained (consumption) in rats.

METHODS

Two groups of rats were trained to press levers with food (45-mg pellet) or cocaine (0.32 mg/kg/injection) reinforcement under fixed-ratio (FR) 5-response schedules. In selected sessions, the FR requirement was increased (5-80) during successive 20-min components to determine demand curves, which plot consumption against price. An exponential function was fitted to the data to derive the consumption at zero price (Q ₀) and the rate of decrease in consumption (essential value, EV) with increased price. The BZT analogs, AHN1-055, AHN2-005, JHW007 (3.2-10 or 17.8 mg/kg, each), vehicle, or comparison drugs (methylphenidate, ketamine), were administered i.p. before selected demand-curve determinations.

RESULTS

Consumption of cocaine or food decreased with increased FR requirement. Each drug shifted the demand curve rightward at the lowest doses and leftward/downward at higher doses. The effects on EV and Q ₀ were greater for cocaine than for food-reinforced responding. Additionally, the effects of the BZT analogs on EV and Q ₀ were greater than those obtained with a standard dopamine transport inhibitor, methylphenidate, and the NMDA antagonist, ketamine (1.0-10.0 mg/kg, each). With these latter drugs, the demand-curve parameters were affected similarly with cocaine and food-maintained responding.

CONCLUSIONS

The current findings, obtained using a behavioral economic assessment, suggest that BZT analogs selectively decrease the reinforcing effectiveness of cocaine.

Heroin and saccharin demand and preference in rats

BACKGROUND

Several recent studies have investigated the choice between heroin and a non-drug alternative reinforcer in rats. A common finding in these studies is that there are large individual differences in preference, with some rats preferring heroin and some preferring the non-drug alternative. The primary goal of the present study was to determine whether individual differences in how heroin or saccharin is valued, based on demand analysis, predicts choice.

METHODS

Rats lever-pressed for heroin infusions and saccharin reinforcers on fixed-ratio schedules. The essential value of each reinforcer was obtained from resulting demand curves. Rats were then trained on a mutually exclusive choice procedure where pressing one lever resulted in heroin and pressing another resulted in saccharin. After seven sessions of increased access to heroin or saccharin, rats were reexposed to the demand and choice procedures.

RESULTS

Demand for heroin was more elastic than demand for saccharin (i.e., heroin had lower essential value than saccharin). When allowed to choose, most rats preferred saccharin. The essential value of heroin, but not saccharin, predicted preference. The essential value of both heroin and saccharin increased following a week of increased access to heroin, but similar saccharin exposure had no effect on essential value. Preference was unchanged after increased access to either reinforcer.

CONCLUSION

Heroin-preferring rats differed from saccharin-preferring rats in how they valued heroin, but not saccharin. To the extent that choice models addiction-related behavior, these results suggest that overvaluation of opioids specifically, rather than undervaluation of non-drug alternatives, could identify susceptible individuals.

Dose and elasticity of demand for self-administered cocaine in rats

The present experiment tested whether the elasticity of demand for self-administered cocaine in rats is dose-dependent. Subjects lever pressed for three different doses of intravenous cocaine - 0.11, 0.33, and 1.0 mg/kg/infusion - on a demand procedure where the number of lever presses required per infusion increased within a session. The main finding was that demand for the 0.11 mg/kg dose was more elastic than it was for the two larger doses. There was no difference in demand elasticity between the 0.33 and 1.0 mg/kg doses. These results parallel findings previously reported in monkeys. The present study also demonstrated that a within-session procedure can be used to generate reliable demand curves.

Characterizing the relationship between increases in the cost of nicotine and decreases in nicotine content in adult male rats: implications for tobacco regulation

BACKGROUND

A large reduction in the nicotine content of cigarettes may benefit public health by reducing the rate and the prevalence of smoking. A behavioral economics framework suggests that a decrease in nicotine content may be considered an increase in the unit price of nicotine (unit price = reinforcer cost/reinforcer magnitude). Increasing the price of cigarettes (i.e., increasing reinforcer cost) would be considered an equivalent change in unit price to reducing nicotine content (i.e., reducing reinforcer magnitude).

OBJECTIVES

The goal of the present experiments was to characterize the relationship between increases in nicotine cost and decreases in nicotine dose.

MATERIALS AND METHODS

A rat self-administration model was used to assess this relationship across three experiments, with an emphasis on very low nicotine doses to model a potential nicotine reduction policy. Cost was manipulated via changes in the number of responses required to earn an infusion.

RESULTS

Results show that increases in the cost of nicotine and decreases in nicotine content were not equivalent manipulations. Nicotine consumption was more sensitive to nicotine dose than to nicotine cost. Nicotine consumption was also not equivalent across a variety of cost and dose combinations forming a single unit price.

CONCLUSIONS

Results of the present studies suggest that nicotine reduction is likely to have a large impact on nicotine exposure from cigarettes.

A generalized matching law analysis of cocaine vs. food choice in rhesus monkeys: effects of candidate 'agonist-based' medications on sensitivity to reinforcement

BACKGROUND

We have previously demonstrated reductions in cocaine choice produced by either continuous 14-day phendimetrazine and d-amphetamine treatment or removing cocaine availability under a cocaine vs. food choice procedure in rhesus monkeys. The aim of the present investigation was to apply the concatenated generalized matching law (GML) to cocaine vs. food choice dose-effect functions incorporating sensitivity to both the relative magnitude and price of each reinforcer. Our goal was to determine potential behavioral mechanisms underlying pharmacological treatment efficacy to decrease cocaine choice.

METHODS

A multi-model comparison approach was used to characterize dose- and time-course effects of both pharmacological and environmental manipulations on sensitivity to reinforcement.

RESULTS

GML models provided an excellent fit of the cocaine choice dose-effect functions in individual monkeys. Reductions in cocaine choice by both pharmacological and environmental manipulations were principally produced by systematic decreases in sensitivity to reinforcer price and non-systematic changes in sensitivity to reinforcer magnitude.

CONCLUSIONS

The modeling approach used provides a theoretical link between the experimental analysis of choice and pharmacological treatments being evaluated as candidate 'agonist-based' medications for cocaine addiction. The analysis suggests that monoamine releaser treatment efficacy to decrease cocaine choice was mediated by selectively increasing the relative price of cocaine. Overall, the net behavioral effect of these pharmacological treatments was to increase substitutability of food pellets, a nondrug reinforcer, for cocaine.

Effect of experimental analogs of contingency management treatment on cocaine seeking behavior

BACKGROUND

Contingency management (CM) treatment is effective for treating cocaine dependence but further mechanistic studies of its efficacy are warranted. This study aimed to determine whether: (a) higher vs. lower predictable money amounts ($3 vs. $1; analogs of standard voucher-based CM) increase cocaine demand elasticity; and (b) probabilistic amounts matched for expected value with the $3-predictable amount (50% chance of $6; 25% chance of $12; and 12.5% chance of $24; analogs of prize CM) similarly affect cocaine choice.

METHODS

Each of 15 cocaine-dependent participants first completed a qualifying session to ensure that intranasal cocaine functioned as a reinforcer, then completed a 10-session, within-subject, randomized crossover study. During each of the 10 sessions, the participant responded on a progressive ratio schedule to earn units of cocaine (5-mg or 10-mg) and/or money (five monetary conditions above).

RESULTS

During the reinforcement qualifying session (10-mg vs. 0-mg units; no money alternative), cocaine choice was high. The $3-predictable amount significantly decreased cocaine choice relative to both the $1-predictable amount and the qualifying session. Cocaine-choices in the probabilistic conditions were similar to the $3 predictable condition.

CONCLUSIONS

These findings indicate that CM interventions targeted at reducing cocaine self-administration are more likely to succeed with higher value non-drug reinforcement.

Peer influences on drug self-administration: an econometric analysis in socially housed rats

Social-learning theories of substance use propose that members of peer groups influence the drug use of other members by selectively modeling, reinforcing, and punishing either abstinence-related or drug-related behaviors. The objective of the present study was to examine the social influences on cocaine self-administration in isolated and socially housed rats, under conditions where the socially housed rats were tested simultaneously with their partner in the same chamber. To this end, male rats were obtained at weaning and housed in isolated or pair-housed conditions for 6 weeks. Rats were then implanted with intravenous catheters and cocaine self-administration was examined in custom-built operant conditioning chambers that allowed two rats to be tested simultaneously. For some socially housed subjects, both rats had simultaneous access to cocaine; for others, only one rat of the pair had access to cocaine. An econometric analysis was applied to the data, and the reinforcing strength of cocaine was measured by examining consumption (i.e. quantity demanded) and elasticity of demand as a function of price, which was manipulated by varying the dose and ratio requirements on a fixed ratio schedule of reinforcement. Cocaine consumption decreased as a function of price in all groups. Elasticity of demand did not vary across groups, but consumption was significantly lower in socially housed rats paired with a rat without access to cocaine. These data suggest that the presence of an abstaining peer decreases the reinforcing strength of cocaine, thus supporting the development of social interventions in drug abuse prevention and treatment programs.

A novel IV cocaine self-administration procedure in rats: differential effects of dopamine, serotonin, and GABA drug pre-treatments on cocaine consumption and maximal price paid

RATIONALE

Behavior occurring during cocaine self-administration can be classified as either consummatory or appetitive. These two concepts are usually addressed independently using separate reinforcement schedules. For example, appetitive behavior can be assessed with a progressive ratio schedule, whereas consummatory behavior is typically measured using a fixed ratio schedule.

OBJECTIVES

Depending on the schedule used, it is often difficult to determine whether a particular drug pretreatment is affecting self-administration through an effect on appetitive responding, consummatory responding, or perhaps both. In the present study, we tested the effect of pretreating rats with four different drugs on appetitive and consummatory behaviors.

MATERIALS AND METHODS

We recently developed a technique that provides an independent assessment of both behavioral concepts within the same experimental session. In this threshold procedure, rats are offered a descending series of 11 unit doses (422-1.3 μg/injection) during consecutive timed intervals under a fixed-ratio schedule. Consummatory behavior can be analyzed by assessing intake at high unit doses; an estimate of appetitive responding can be determined from responding occurring at the threshold dose. Applying behavioral economics to these data provides dependent measures of consumption when minimally constrained by price and the maximal price paid (P (max)) for cocaine.

RESULTS

Haloperidol increased cocaine consumption when minimally constrained by price but decreased P (max). In contrast, D: -amphetamine increased P (max). Fluoxetine decreased P (max) and consumption when minimally constrained by price. Baclofen selectively decreased P (max).

CONCLUSIONS

These data suggest that drug pretreatments can alter consummatory and appetitive behavior differently because each concept involves distinct neural mechanisms.

Effects of altering reinforcer magnitude and reinforcement schedule on phencyclidine (PCP) self-administration in monkeys using an adjusting delay task

Compared to nondrug reinforcers, few studies have examined delay discounting for drug reinforcers. The purpose of the present study was to examine delay discounting in rhesus monkeys using orally delivered phencyclidine (PCP) as the reinforcer and to examine the effects of manipulating reinforcer magnitude and cost on delay discounting for PCP using an adjusting delay task. Monkeys could choose between a single delivery of PCP available immediately or a bundle of PCP deliveries available following a titrated delay. The average of the delays, or the mean adjusted delay (MAD), served as the quantitative measure of delay discounting. In Experiment 1, reinforcer magnitude was manipulated by varying the PCP concentration and the size of the delayed reinforcer (6 or 12 deliveries). The concentration-effect curve for PCP deliveries assumed an inverted U-shaped function, but varying PCP concentration had little effect on MAD values or the choice between immediate and delayed reinforcers. Increasing the size of the delayed reinforcer produced an upward and leftward shift in the concentration-effect curve. In Experiment 2, the cost of reinforcers was manipulated by increasing the fixed ratio (FR) requirement for each choice. Increasing the FR led to increased MAD values and decreased PCP self-administration.

Behavioral economic analysis of opioid consumption in heroin-dependent individuals: effects of unit price and pre-session drug supply

Behavioral economic analysis has been used to investigate factors underlying drug consumption in laboratory animals and, increasingly, in human drug abusers. However, there are few studies in heroin abusers, especially those who are not in treatment; such studies may be valuable for understanding the mechanisms of persistent drug use. This study investigated effects of unit price (UP) and pre-session supply of hydromorphone (HYD) on choice and consumption of HYD. Heroin-dependent research volunteers (n=13) stabilized on buprenorphine 8 mg/day completed this eight-session inpatient study. In sessions 1-2, participants sampled two total HYD doses (12 and 24 mg IM) that could be earned in later sessions. In each of the final six sessions, volunteers were given access to a 12-trial choice progressive ratio schedule lasting 3h. On each trial, volunteers could earn a HYD unit dose (1 or 2 mg, for a maximum of 12 or 24 mg, respectively) or money (US dollars 2, for a maximum of US dollars 24). Fixed ratio requirements increased exponentially, generating 24 unit prices for behavioral economic analysis. Before some choice sessions, volunteers could choose (FR 1) to receive extra HYD (12 or 24 mg; at 0915), whereas on other days no supplement was available. HYD choice and peak responding (breakpoint, O(max)) measures increased with unit dose, decreased with pre-session supplements, and were greater among volunteers who used cocaine prior to the experiment. Taking pre-session supplements decreased P(max) and made group-percent HYD consumption more demand-elastic. Consumption was functionally equivalent at differing FR/unit dose combinations. Thus, opioid demand in heroin-dependent individuals not in treatment is a function of drug supply, unit price, and cocaine use.

Labor supply and consumption of food in a closed economy under a range of fixed- and random-ratio schedules: tests of unit price

The behavioral economic concept of unit price predicts that consumption and response output (labor supply) are determined by the unit price at which a good is available regardless of the value of the cost and benefit components of the unit price ratio. Experiment 1 assessed 4 pigeons' consumption and response output at a range of unit prices. In one condition, food was available according to a range of fixed-ratio schedules, whereas in the other condition, food was available according to a range of random-ratio schedules. Consistent with unit price predictions, consumption and response output were approximately equivalent across schedule types within the lower range of unit prices. However, at Unit Prices 64 (ratio value = 192) and greater, considerably more consumption and response output were observed in the random-ratio condition. Experiment 2 replicated these findings with 4 pigeons using the rapid demand curve assay procedure that is commonly used in the behavioral economics literature. Findings are integrated with two mathematical models of behavior under variable reinforcer delays.

Three predictions of the economic concept of unit price in a choice context

Economic theory makes three predictions about consumption and response output in a choice situation: (a) When plotted on logarithmic coordinates, total consumption (i.e., summed across concurrent sources of reinforcement) should be a positively decelerating function, and total response output should be a bitonic function of unit price increases; (b) total consumption and response output should be determined by the value of the unit price ratio, independent of its cost and benefit components; and (c) when a reinforcer is available at the same unit price across all sources of reinforcement, consumption should be equal between these sources. These predictions were assessed in human cigarette smokers who earned cigarette puffs in a two-choice situation at a range of unit prices. In some sessions, smokers chose between different amounts of puffs, both available at identical unit prices. Individual subjects' data supported the first two predictions but failed to support the third. Instead, at low unit prices, the relatively larger reinforcer (and larger response requirement) was preferred, whereas at high unit prices, the smaller reinforcer (and smaller response requirement) was preferred. An expansion of unit price is proposed in which handling costs and the discounted value of reinforcers available according to ratio schedules are incorporated.

Preclinical research on cocaine self-administration: environmental determinants and their interaction with pharmacological treatment

It has been asserted that any comprehensive understanding of cocaine abuse and its treatment will require attention to both behavioral and pharmacological variables. Although the preclinical literature evaluating the effects of pharmacological variables on cocaine self-administration has been extensively reviewed, no comprehensive review of the effects of environmental variables on cocaine self-administration has been published. The present review summarizes and critiques the preclinical findings on environmental determinants of cocaine self-administration. The influence of environmental variables on the effects of pharmacological interventions on cocaine self-administration are also described. Several environmental variables have been shown to affect cocaine self-administration, including unit dose, schedule of cocaine delivery, schedules of nondrug stimuli, behavioral history, conditioned stimuli, food deprivation, exposure to stress, and rearing environment. Among these variables, unit dose, schedule of cocaine delivery, availability of alternative nondrug reinforcers, food deprivation, and rearing environment have also been shown to alter pharmacological treatment effects on cocaine self-administration. Thus, drug effects on cocaine self-administration are malleable and dependent upon the environmental context within which they occur. Suggestions for future research on the effects of these and other environmental variables on cocaine self-administration and its pharmacological treatment are presented.

Ratio size and cocaine concentration effects on oral cocaine-reinforced behavior

Monkeys were given a choice between cocaine solutions and water under concurrent fixed-ratio reinforcement schedules. The operant response was spout contact. Six rhesus monkeys served as subjects. The cocaine concentration was varied from 0.0125 to 0.8 mg/ml, and the fixed-ratio value was varied from 8 to 128. Cocaine maintained higher response rates than did water over a wide range of conditions. Response rate and number of cocaine deliveries per session were inverted U-shaped functions of concentration. These functions were shifted to the right as the fixed ratio was increased. The number of cocaine deliveries was more persistent as fixed-ratio value was increased when the unit dose was larger rather than smaller. Cocaine consumption was analyzed as a function of unit price (fixed-ratio value divided by cocaine concentration), and unit price accounted for between 77% and 92% of the variance in cocaine consumption for individual monkeys. The current data support the claim that a drug's reinforcing effects increase directly with dose and underscore the need to gather parametric data when examining the effects of experimental manipulations on a drug-reinforced baseline.

Further analysis of choice between cocaine and food using the unit price model of behavioral economics

Behavioral economics defines unit price (UP) as the ratio of the response requirement to magnitude of reinforcer. When applied to drug self-administration, the UP model defines UP as the ratio of the response requirement to the unit dose of drug. This model makes two predictions about drug self-administration: increasing UP decreases consumption and consumption at a given UP will be constant regardless of the response requirement and dose that make up the UP. In previous experiments conducted in rhesus monkeys allowed to choose between an i.v. injection of cocaine and food, the UP model has failed to adequately predict drug consumption in that consumption varied (increased with dose) at a given UP. However, previous experiments have allowed a fixed number of choice trials/day, thereby imposing a procedural ceiling on consumption that may have influenced conformity to the UP model. In the present experiment, the number of choice trials available was varied in such a way that constant drug consumption was possible over the range of UPs tested. The response requirement for cocaine was varied between 15 and 1200 lever presses/injection and the dose of cocaine was varied between 0.05 and 0.2 mg/kg/inj, yielding UPs from 300 to 5600 responses/mg/kg. The response requirement for food was always 30. As predicted by the UP model, cocaine consumption decreased as UP increased. Moreover, in contrast to previous experiments, consumption did not vary significantly across the response requirement/dose combinations that made up a UP. A detailed analysis suggested that a decrease in magnitude of the alternative reinforcer (one rather than three food pellets), rather than the increase in trials, was responsible for the improved conformity to the UP model in the present experiment relative to previous experiments. Taken together with previous experiments, the present experiment suggests that conformity to the UP model of drug consumption in a choice situation is dependent upon the magnitude of alternative reinforcers that are available. Consumption was best predicted by the UP model when the magnitude of the alternative reinforcer was small.

Associations between drug use and behavioral repertoire in urban youths

Converging with psycho-social research findings, animal and human laboratory studies indicate that behavioral alternatives are important determinants of drug-taking. To investigate associations between how early adolescents spend their time, i.e. their behavioral repertoire and drug use (use of marijuana, crack/cocaine or inhalants), we analyzed data from an epidemiological sample of 1516 urban middle-school students who had completed private interviews in spring 1993. The interview included a 36-item questionnaire to assess how frequently the youth engaged in different activities; history of drug-taking was assessed separately. Multiple logistic regression was used to estimate associations between drug use and each of seven behavioral domains as well as sex, age and racial-ethnic status. Youths spending a great deal of time working for pay and assuming other adult-like roles were more likely to have initiated drug use (estimated odds ratio, OR = 3.49; p = 0.002). Those who spent much time in religious activities were less likely (OR = 0.2, p <0.001). An exploratory search for interactions disclosed other associations that merit attention in future research. These results corroborate evidence on the potential etiological significance of behavioral repertoire in relation to risk of drug use.

Smoked cocaine self-administration in females

Although approximately 32% of all smoked cocaine ("crack") users are women, most studies investigating cocaine use have recruited only male subjects. Therefore, this study examined the smoked cocaine self-administration patterns of female crack abusers. A work requirement, where up to five tokens were earned by subjects, was followed by the administration of a sample delivery of one of three dose sizes [5.0 mg (placebo), 0.2 mg/kg, 0.4 mg/kg] of cocaine. The three dose sizes of cocaine were administered in counterbalanced order across subjects, with each subject receiving one dose size within a session and all dose sizes across the three experimental sessions. A self-administration phase followed the sample delivery, during which a token could be exchanged every 30 min for monetary reimbursement or a delivery of cocaine in the same dose size as the sample delivery. The results show that females' patterns of behavioral and subjective responding to smoked cocaine exhibit clear dose-related effects, thus affirming this self-administration model as safe and appropriate for use with women.

Unit-price analysis of opioid consumption by monkeys responding under a progressive-ratio schedule of drug injection

Several reports have indicated that drug consumption in self-administration procedures is a function of the ratio of the instrumental requirement to the dose of drug, a quantity termed unit price. We evaluated three predictions from this unit-price model in a reanalysis of data on opioid self-administration in rhesus monkeys responding under a progressive-ratio schedule (Hoffmeister, 1979). We evaluated whether consumption was inversely related to unit price, and compared the goodness of fit of an equation devised by Hursh, Raslear, Shurtleff, Bauman, and Simmons (1988) to that of a linear model predicting consumption as a function of dose. We also tested whether consumption was constant when the same unit price was comprised of different combinations of dose and instrumental requirement. Consumption declined overall as unit price increased. The equation devised by Hursh et al. and the linear model based on dose fit the data equally well. Drug consumption was not uniform at a given unit price. The analyses suggest limits on the unit-price model as a characterization of drug consumption.